A seal ring used e.g. in hydraulic units of an automatic transmission or a continuously variable transmission for sealing an annular gap between a housing and a shaft member rotatably mounted in the housing is fitted in an annular groove formed in the radially outer surface of the shaft member, and pressed against a side wall of the annular groove near a non-oil pressure side and a radially inner surface of the housing that opposes the annular groove, thereby partitioning the annular gap into an oil pressure side and the non-oil pressure side.
Many of such seal rings are made of a resin, and cut apart at one circumferential point thereof to define circumferentially opposed abutments. A circumferential gap is defined between the abutments to allow for thermal expansion of the seal ring. Each of the abutments comprises a protrusion and a recess that are complementary in shape to and in engagement with the recess and the protrusion of the other abutment, respectively. In order to reduce friction and frictional heat buildup between the side wall of the annular groove near the non-oil pressure side and the side of the seal ring which is in sliding contact with the side wall of the annular groove, an oil passage is formed between the opposed abutments which communicates with both sides of the annular gap of the seal ring to allow oil in the oil pressure side of the annular gap to leak slowly into the non-oil pressure side of the annular gap, thereby supplying oil between the side wall of the annular groove and the side of the seal ring that is in sliding contact with the side wall of the annular groove, and simultaneously discharging wear dust contained in oil in the oil pressure side into the non-oil pressure side (see e.g. Patent documents 1 and 2).
The seal ring disclosed in Patent document 1 has a crank-shaped oil passage defined between abutments at its radially outer portion. The oil passage comprises a first axial oil groove extending to one side of the seal ring, a second axial oil groove extending to the other side of the seal ring, and a third circumferential oil groove connecting the first and second oil grooves together.
The seal ring disclosed in Patent document 2 is formed with a groove along the ridge where two adjacent ones of the outer wall surfaces of the protrusions of the abutments intersect each other. The groove has a polygonal or fan-shaped section in the direction of the ridgeline. An oil passage between the abutments is partially defined by gaps between the protrusions each formed with the above-described groove and the corners of recesses in engagement with these protrusions. Specifically, each of the grooves forming the gaps between the protrusions and the corners of the recesses are provided along a radial ridgeline where the front end surface of the protrusion intersects the outer side surface, an axial ridgeline where the front end surface intersects the radially inner surface, and a circumferential ridgeline where the radially inner surface intersects the inner side surface.    Patent document 1: JP Patent Publication 8-135797    Patent document 2: JP Patent Republication WO2003/078873
In the seal ring disclosed in Patent document 1, because the crank-shaped oil passage is partially formed by the circumferentially extending third oil groove defined between the inner side surface of one of the protrusions and the other abutment, the protrusions, which are made of a resin, tend to be deflected toward the inner side surface under the oil pressure of the oil pressure side, thereby narrowing the width of the third oil groove, or closing it. If this happens, foreign matter contained in oil on the oil pressure side may clog the third oil groove, thereby making it difficult to smoothly discharge foreign matter into an annular space on the non-oil pressure side.
In the case of the seal ring disclosed in Patent document 2, because grooves are formed along many ridgelines of the protrusions of the abutments, the abutments are complicated in shape. Also, because the oil passage formed by gaps defined between these grooves and the corners of the recesses have many bent portions, foreign matter contained in oil in the oil pressure side tends to clog the oil passage at these bent portions.